Matsumoto or Hida Takayama?

Which place to visit, if I could only go to one: Matsumoro or Hida Takayama?
I am trying to travel only by train, not bus, I have a car sick child. Thank you

If you want to see a nice castle - visit Matsumoto

If you want to see an old town - visit Takayama

Note that Matsumoto castle (the castle itself) has some of the steepest stairs you will ever see. So to put it simple - the castle itself is not pram or little kid friendly, but it is a couple of hundred years old so they did not think of that when building.....

Everything else is really up to you as you've told us little about what you like apart from traveling with kid and by train.

If you base in Matsumoto, you can see the old town section along the river as well as the castle. If you have a train pass, it wouldn't be difficult to ride down to Narai-juku to see an old post town or Kiso-Hirasawa to see a center for lacquerware production. You can also get off in Kiso-Hirasawa and walk following the river down to Narai-juku as it's not that far.

There is also a lot to do in Hida-Takayama. There's the old town section, the Hida Folk Village with old houses brought from the surrounding area and some crafts being shown in some of the buildings (shingle making, sandal making, wood carving, etc.). There are also a lot of museums, including a teddy bear Eco Village depending on the age of your kid, as well as the festival float museum, a lacquerware museum, a "lion's head" museum and others. There is also a temple walk along the edge of the hills on the east side of town.

I've been to matsumoto three times as a rest night and the first time to stay for a few days, and really except for the weekends it isn't a very lively town. Event the castle is hard to get into these days as it seems to ve on tge map for chinese tourist buses.

I've been to takayama 4 times for a total of maybe 10 days, I still make a point of visiting it. It is just a nice town. The squirrel park is awesome too.